Sorry Makedo626, I thought I was in some kind of a live chat, thanks for responding, I've just posted a much in depth analysis of my problem. No messed pixels, reboot very random but more likely when system is under load directories open, playing videos, no games, but this is not 100 pct can reboot with very little load.
Computer starts very well, computer has been up and running now for 48 hours no reboot but open a video will reboot in minutes. Sitting there quite happily now no reboot so long as I do nothing on the computer. So yes it stays on until I do something. Single normal beep at the start of reboot. Reboots perfectly, most times, a few times seems to get itself lost, press restart and starts fine, a couple of times in the past 4 months I've had serious problems with starting. There seems to be a whole collection of problems here rather than one pattern occurance. No error text before rebooting. Sorry about my original post.
Thanks for your time